The present invention relates to a semiconductor infrared emitting device for use in a 2 to 3 .mu.m wavelength region.
The transmission loss of an optical silica fiber has now been reduced down to 0.2 dB/km in a 1.5 .mu.m region, which is substantially equal to a theoretical limit. On the other hand, study is being given a fluoride glass fiber whose transmission loss is expected to be 1 to 2 orders of magnitude lower than that of the optical silica fiber. The fluoride glass fiber exhibits a low transmission loss in the 2 to 3 .mu.m region, wherein a very low loss of 0.01 to 0.001 dB/km is anticipated. It is now being considered that a non-repeated optical transmission system over 1000 km will become a reality in the future through utilization of the fluoride glass fiber in combination with light emitting and receiving devices intended for use in the 2 to 3 .mu.m region.
A semiconductor laser is considered excellent, as a light emitting device for use in the 2 to 3 .mu.m region, in terms of operability and reliability. As infrared semiconductor lasers for use in this wavelength region there have been studied so far a semiconductor laser which has an active layer formed of GaInAsSb lattice-matched with a GaSb substrate and a semiconductor laser which has an active layer formed of InAsPSb lattice-matched with an InAs substrate. From the viewpoint of the operating temperature range the infrared semiconductor laser which includes the GaInAsSb active layer and AlGaAsSb clad layers on the GaSb substrate has been developed most, and its pulse oscillation at room temperature has been reported. With the use of semiconductor materials of this series, however, it is very difficult to obtain a GaInAsSb active layer composition corresponding to an energy gap of 0.35 to 0.5 eV, because of the presence of a miscibility gap. This indicates difficulty in manufacturing, through use of semiconductor materials of this kind, an infrared semiconductor laser which emits light at a wavelength of 2.5 to 2.7 .mu.m or 3.5 .mu.m where the fluoride glass fiber is considered to exhibit a low loss. In fact, the longest wavelength of light emitted by the infrared semiconductor laser made of this kind of materials so far is about 2.2 .mu.m.
On the other hand, in the infrared semiconductor laser of the InAsPSb alloy formed on the InAs substrate, both the active layer and the clad layer can be formed of the same semiconductor materials and an active layer of a composition which emits light at the 2.5 to 2.7 .mu.m wavelength can also be obtained. However, the semiconductor materials of this series also presents the above-mentioned miscibility gap, which constitutes an obstacle to the formation of a clad layer of a large energy gap. The upper limit of the energy gap of the InAsPSb is estimated to be about 0.6 eV at an absolute temperature of 77K. On this account, in a case where the energy gap of the active layer is selected to be 0.5 eV (corresponding to a 2.5 .mu.m wavelength), the barrier height for carrier confinement, which is defined by a difference in energy gap between the clad layer and the active layer, is only 0.1 eV at the most. As a result of this, a threshold current value for lasing increases, limiting the working temperature to a low temperature range of between 77 and 150K.
Furthermore, a non-radiative recombination process by the Auger effect in the semiconductor forming the active layer becomes marked in the 2 to 3 .mu.m region, and this also constitutes one of the obstacles to a low threshold value and a high-temperature operation.
In the manufacture of an infrared semiconductor laser for operation in the 2 to 3 .mu.m region through use of materials which satisfy the requirement of lattice matching of both the active layer and the clad layer with the substrate as described above, the substrate will be limited to the GaSb or InAs substrate and the active layer will have to be formed of the GaInAsSb or InAsPSb layer. In this instance, however, the above-mentioned problem inherent to the bulk semiconductor still remains unsolved. This makes it difficult to realize an infrared semiconductor laser for fluoride glass fiber optical communication which operates with a low threshold current and at high temperatures.